1. Field of the Invention
The present invention relates to a process and a device for the production of objects or models of industrial parts, by action of light and especially by phototransformation of a plastic or composite material.
2. Description of the Related Art
It is desirable to be able to have available a few models of an industrial part such as, especially, a mechanical part, before initiating the mass production thereof. At the present time, in order to construct models of such parts, it is necessary in the first instance to arrange for the production of dimensioned drawings in the design office on the basis of numerical data which define the shape of the part. For the past few years, the method of "computer-aided design" (CAD) has been employed for the purpose of defining the shape of this part.
It is known to provide for a process and for a device for the production of models of industrial parts making use of the numerical data available in the output memories of the computer, directly for the purpose of constructing the models of parts without being obliged to proceed via drawings or to have to involve numerical-control finishing machines such as milling machines, lathes, etc.
Processes and devices proposed for the construction of models by means of CAD are based on a LASER-induced chemical polymerisation. The slaved displacement of one or more laser beams concentrated at a single point permits the local polymerization of a polyfunctional monomer and, step by step, the construction of the part.
In particular, the document FR-A-2,567,668 describes a process, of which the general principle of construction of a part is based on the use of an apparatus comprising a cell which contains the liquid photopolymerizable monomer at the location of which there is aligned a movable platform which descends step by step in the course of the operation. The UV laser beam traces at the surface of the liquid successive sections of the object to be produced, by virtue of a set of mirrors with electronic deflection, which set is driven by the database of the CAD system. The software has previously subdivided the virtual model of the object into a multitude of slices of small thickness. Progressively as the solidification of each slice proceeds, the "carved" model is slowly immersed in the cell: using a 10 mW laser, this system permits the creation of a part having a height of 30 mm in 50 minutes, with an accuracy of the order of 1/10 mm. The model may be created in all types of materials (plastics), in a wide variety of colors, with differing hardnesses and a whole range of resistances to abrasion (SCIENCE ET TECHNOLOGIE No. 2, February 1988/INDUSTRIAL LASER REVIEW, Vol. 2, No. 8, January 1988, pages 11 and 13). This type of process for the manufacture of three-dimensional models, which makes use of the principle of the single-photon absorption of light by polymerization or cross-linking photoinitiators, exhibits the disadvantage of requiring the superposition of the monomer (or of an oligomer or of a mixture) on parts which have already been polymerized; in order to obtain a good accuracy of the model, it is necessary to superpose a considerable number of liquid or pasty layers of monomer; this may lead to fairly long construction times. Furthermore, the material obtained after irradiation has, in general, a greater density than the initial material; this may lead to difficulties in carrying out the process and to instabilities, deformations or stresses on the whole or part of the model.
The document EP-A-O,250,121 describes a process corresponding to the same general principle.
French patent application No. 2,639,948, filed by the National Centre for Scientific Research and not published after the date of priority of the present application, proposes a process permitting the production of models of industrial parts by action of light, giving improved compliance with the practical requirements as compared with the processes and devices aiming at the same object proposed in the prior art.
In this process, a part is constructed by the superposition of a plurality of layers of a solid or plastic material which is made either soluble in the presence of light or insoluble in the presence of a light of appropriate wavelength.
After successive irradiation of each layer, at appropriate locations, it is sufficient to place in a bath of appropriate solvent the stack of layers, which are linked to one another at the appropriate sites.
This method exhibits the following advantages:
"dry" and odorless manufacture of a part;
creation of a three-dimensional latent image permitting in principle the use of a CAD which is simplified as compared with that which is required for photochemical polymerizations of liquids, involving the need to anchor the parts on the support.
However, the method exhibits limitations:
The first corresponds to the existence of a shrinkage associated with the formation of chemical bonds, occupying a smaller space between the insoluble cross-linked polymers.
The second is the difficulty of removing the non-irradiated polymer (if it is this which is soluble) from the irradiated polymer without loss of accuracy of the patterns. The concept of solubility/insolubility is theoretical; actually, only relative differences of solubility are involved. For parts in which the non-irradiated polymer is easily accessible to the solvent, which is itself liable to be well stirred, permitting a promoted dissolving, the insoluble irradiated polymer is virtually not attacked at all.
On the other hand, for hollow parts, in which the dissolving cannot be promoted by the stirring, the time of contact of the solvent may be so long that a part of the irradiated polymer may be dissolved. It is, of course, the same if it is the non-irradiated polymer which is "insoluble".
These two limitations permit the production of patterns corresponding precisely to what is specified only when these patterns comply with certain conditions, and it is difficult and costly to modify the programs in order to take account in advance of the influence of the shrinkages or of the non-homogeneous dissolvings.
Materials are known which have a very low shrinkage, and even materials which exhibit a swelling under irradiation, so that, by mixing, it is possible to obtain a zero shrinkage. However, the use of such mixtures results in the replacement of the limitations inherent in a shrinkage by limitations relating to the selection of the materials.